The present invention relates to controls for the cycling of electrical heaters as, for example, in an oven, to maintain a regulated temperature environment in the space to be heated. The invention particularly relates to control systems for temperature regulation of the type employed in household cooking ovens utilizing electrical resistance heating elements. Typically, in such cooking ovens, a thermostatic switch senses the temperature of the air in the oven and turns the heating element on or off, depending on whether the sensed temperature is above or below the user-selected regulation temperature.
In the interests of safety, a separate limit control switch is series connected with the thermostatically controlled switch to provide cutoff of the heater current in the event of a malfunction in the thermostatic switch.
The thermostatic switches, which are typically bimetal devices, require a user selection of a precalibrated spring bias thereon for selection of the temperature about which the oven is to be regulated; and, it has been desired to eliminate such switches because of the volume of the switches and the space limitations at the control or mounting panel.
In recent times, it has been desired to provide an all-electronic controller for electrical oven or space heaters and to thereby eliminate the expense and space requirements for thermostatic switches for selecting the temperature of regulation. However, where solid state electronic switching components are employed, the limitations of the low voltage, low current handling capabilities of the solid state electronic components has necessitated the use of electromechanical relays for switching the load current to the heating elements such as the BAKE and BROIL elements in a household cooking oven. Where bimetal thermostats are employed instead of solid state controlling components, the bimetal thermostat is capable of performing the same switching function as the electromechanical relay; and, the thermostatically controlled type system thus has a manufacturing cost advantage over the electronic oven control system.
However, the increased functional capability and sophistication of electronic microcomputers has brought about a certain market demand for electronically controlled appliances, and it has thus been desired to provide electronic control of oven heating elements despite the necessity of employing electromechanical relays to energize and de-energize the oven heaters. Where bimetallic thermostatically controlled switches have been employed for oven temperature regulation, it has been a simple matter of providing an additional such thermostatic switch as an over-temperature or limit control to provide a backup in the event of failure of the oven regulation thermostatic switch; and, this technique has proven cost-effective for providing oven temperature limiting safety switches.
However, where an all electronic oven control system is employed, particularly a system utilizing a microcomputer, it is required to provide a separate temperature limiting switching means to provide for high temperature cutoff of the heating elements in the event of failure of the microcomputer. Accordingly, it has been desired to provide a simple yet reliable and cost-effective way or means of providing over-temperature limit control in an electronic control system for cycling electronic heaters to maintain a regulated temperature in a space to be heated, and particularly in household cooking ovens.